High-side switch with depletion-mode device

ABSTRACT

A technique for supplying drive voltage to the gate of a high-side depletion-mode N-channel MOS-device for high-side switches or any circuit with a depletion-mode N-channel MOS-device with its source at a voltage above local ground.

This application claims the benefit of Provisional application Ser. No. 60/123,910, filed Mar. 10, 1999.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to power drive circuits using active semiconductor devices.

Background: “MOS” Transistors

One of the basic types of transistors is the insulated-gate field-effect transistor, which is commonly also referred to as a “MOS” transistor or “MOSFET”. In any MOS transistor, the voltage on the “gate” terminal controls the density of majority carriers (electrons or holes) in a semiconducting “channel” region which is part of the conduction path. By controlling the number of majority carriers in the channel, the flow of current from the “source” through the channel and to the “drain” is controlled. Thus a MOS transistor operates as a switch which is controlled (at least partially) by the voltage on its gate.

A “MOSFET” may be configured to be a lateral, vertical, or grooved vertical structure. There are four regions in a MOSFET: source, drain, body, and gate. The source is the region where carriers originate when current is flowing in the transistor in normal operating conditions. The drain is the region where the carriers terminate when current is flowing in the transistor in normal operating conditions. The body is the region whose surface is inverted to form a “channel” when carriers flow in the transistor in normal operating conditions. The gate is an electrode that covers the surface of the body region between the source and the drain regions, and controls the carrier flow between the source and the drain of the transistor in normal operating conditions.

Background: N-Channel Versus P-Channel

MOSFETs are either n-channel devices (in which the majority carriers are electrons) or p-channel devices (in which the majority carriers are holes). The relative performance of equivalent n-channel and p-channel MOSFETs is determined primarily by the “mobility” of the electrons and holes in the inverted “channel” region at the surface of the body region. Electrons have a higher mobility, so the resistance of an n-channel MOSFET is lower than a p-channel MOSFET with the same geometry. (For example, in silicon at room temperature the electron mobility is about 2 to 3 times the hole mobility, and the ratio of mobilities is even larger in gallium arsenide; but in some semiconductors, such as germanium or gallium phosphide, the hole mobility is more nearly equal to the electron mobility.

N-channel transistors also differ from P-channel transistors in some other ways which can be important for power devices. For example, the transient behavior at turn-on and turn-off can be slightly different. This difference can be important for power devices, where mismatch during turn-on (or during turn-off of an inductive load) may lead to disastrous side-effects. In any case, N-channel and P-channel MOS-gated devices rarely have the same switching characteristics for the same geometry.

Background: Overdrive

An N-channel MOS transistor begins to turn “on” when the gate voltage (measured with respect to source voltage) exceeds the threshold voltage (V_(T)). The amount by which the gate voltage exceeds the threshold voltage will be referred to as the “overdrive” voltage. As the overdrive voltage increases, the transistor passes more and more current, up to a “saturation” current value. If the gate voltage is increased further, the current increases much more slowly. However, with power transistors it is common to increase the gate voltage beyond the point where saturation occurs. This gate voltage increase is used to decrease the on-resistance of the transistor as much as possible.

Background: DMOS

A “DMOS” transistor is a particular type of MOS transistor which is commonly used for high-voltage applications. A DMOS device uses a short channel which is defined by differential diffusion, in combination with a drift region which is interposed between the channel and the drain. (The drift region helps to stand off high source-drain voltages when the channel has not been turned “on.”) For high-power applications, one commonly used form of a vertical DMOS is shown in FIG. 1. In the solid structure shown, majority carriers (electrons in this example) flow laterally from source regions 12 through channel regions 14 into drift regions 16, within which the carriers pass vertically to drain 18 on the backside of the semiconductor chip.

MOS-gated devices with their high input impedance were viewed as nearly ideal switches. However, it was the use of the DMOS structure to fabricate power transistors that began a period of significant development in MOS-gated power devices. This evolution is covered in several books and articles, such as “Trends in Power Semiconductor Devices” by B. Jayant Baliga (IEEE Transactions on Electron Devices, Vol. 43, No. 10, October 1996, pp., 1717-1731), Chapter 1 titled “Power Semiconductor Devices for Variable Frequency Drives” written by B. Jayant Baliga in the book, Power Electronics and Variable Frequency Drives edited by Bismal K. Bose (IEEE Press, 1996) and Power Semiconductor Devices by B. Jayant Baliga (PWS Publishing Company, 1996). Other, newer MOS-gated power devices based on DMOS technology such as the insulated-gate thyristors or IGTH (J. S. Ajit and D. M. Kinzer, “1200V, 150A Insulated-Gate Thyristors,” Proceedings of 1995 International Symposium on Power Semiconductor Devices & I.C.s.) and the base open/short-controlled thyristor or BOSCHT have also been developed. (J. S. Ajit, “A New Three-Terminal Thyristor-Based High-Power Switching Configuration with High-Voltage Current Saturation,” IEEE Electron Device Letters, Vol., 18, No., 7, July, 1997.)

Double-diffused MOS or DMOS transistors are MOSFET with a structure that eliminates many of the on-resistance and voltage limitations of conventional MOSFETs. DMOS transistors use the difference in the diffusion of sequentially introduced body and source impurities from a common edge to determine the channel length. Control of both the channel length and the peak dopant concentration in the body are obtained, as in double-diffused bipolar technology, by control of the amount of dopant introduced at the body and source doping step, and by the subsequent diffusion cycle. A DMOS transistor differs from a conventional MOS transistor in two distinct ways:

1. Because the channel Length L is determined by the difference between two sequential diffusions moving in the same direction from a common point of origin, L can be reproducibly controlled to values in the 0.5- to 2-micrometer range.

2. The body region is more heavily doped than the N-drain region, resulting in a junction that depletes further into the drain region than into the body region when a reverse bias is placed across the drain-to-body junction. This difference allows significantly higher voltages to be placed across the body-to-drain junction without markedly affecting the electrical channel length of the transistor.

These two differences in device structure result in MOS transistors with both a short channel length and the ability to withstand high drain-to-source voltages due to the separation of the active or channel region of the device from the region of the device that sustains the drain-to-source voltage. The DMOS transistor structure is somewhat analogous to that used in double-diffused bipolar transistors for many years. (In bipolar transistors, a narrow, moderately doped base region controls device electrical characteristics. A lightly doped N-collector region is used to support applied potentials.)

Background: Depletion-Mode MOS Devices

An N-channel MOS transistor begins to conduct when its gate voltage (measured with respect to its source voltage) rises above a certain “threshold” voltage (which is determined by the compositions, dopings, hand dimensions of any particular device). If the threshold voltage is positive (as is normal), the transistor is normally “off”, and is referred to as an “enhancement-mode” device. Most power MOS transistors have generally been enhancement mode (normally “off”) devices.

However, if the threshold voltage of an N-channel transistor is below zero, the device is referred to as a “depletion-mode” device. In this case the transistor is at least partially “on” with a gate voltage of zero, and the gate voltage must be driven below zero to fully turn the transistor “off.” An advantage of N-channel devices is that it is generally easier to generate a constant negative voltage than to generate a positive voltage that is referenced to the voltage present on the source (or equivalent terminal) of a high-side drive device.

In VLSI devices the threshold voltage is typically controlled by an ion implantation into the semiconductor material of the channel. However, researchers have demonstrated N-channel depletion-mode power transistors, in which the threshold voltage is shifted by an implant into the gate insulator. Even though the gate insulators in power MOS transistors are typically much thicker than in VLSI transistors, implantation into the gate insulator does work well in power MOS devices. See e.g. UK Patent 2,028,528, U.S. Pat. Nos. 3,328,210, 4,774,196, 4,799,100, 4,868,537, 4,958,204, 4,978,631, and U.S. patent application Ser. No. 06/771,444 “Method for Shifting the Threshold Voltage of DMOS Transistors” (referenced in issued U.S. Pat. No. 4,978,631) all of which are hereby incorporated by reference.

The degree of depletion obtained by implantation into the gate insulator is a function of the distance the permanently charged ions are above the gate dielectric-semiconductor interface and the original threshold voltage of the transistor. In an N-channel MOSFET with a gate dielectric consisting of 0.1×10⁻⁶ meter of silicon dioxide, a cesium implant of 3.5×10¹²/cm² is sufficient to move the threshold voltage of a 600V MOSFET from plus 2-4 Volts to minus 6-8 Volts. This change in V_(T) of approximately 10 Volts produces a DMOS transistor that is turned “on” by the permanently ionized impurities in the gate by 6-8 Volts. For the purposes of this disclosure, a transistor that is turned “on” using permanently charged ions by a voltage that is more than twice the unadjusted V_(T) value will be referred to as being in “deep depletion.”

Power DMOS transistors, whether discrete devices, incorporated in integrated circuits, or merged into discrete devices such as IGBTs, MCTs, IGTHs, have almost always been enhancement mode or normally “off” devices. In applications requiring a power switch in series with the load, but in a “low-side” configuration, N-channel power devices are suitable. However, several common power switching configurations (e.g. the high side switch, the half-bridge, and the H-bridge) place different requirements on the switch(es). Looking first at a high-side switch as shown in FIG. 2, an N-channel power MOS transistor or alternatively a MOS-gated power device with a merged N-channel MOSFET is the power device Q_(NE). (The use of a P-channel power MOS transistor or alternatively a MOS-gated power device with a merged P-channel MOSFET is also possible, but the difference in device characteristics that result from the difference between the mobility of conduction electrons and holes, and the resulting cost premium that must be paid for P-channel or P-channel based MOS-gated power devices, often precludes their use.) On the other hand, the use of an N-channel MOS-gated power device (which includes N-channel MOSFETs as well as merged devices based on N-channel MOSFETs) in high-side switch configurations requires a gate voltage that is referenced to the voltage present on the source (or equivalent) terminal S (FIG. 2). If the voltage on the drain (or equivalent) terminal D is the highest voltage in the circuit, a drive voltage that is greater than the voltage on the drain (or equivalent) terminal is almost always needed to turn the switch “on”. In a half-bridge, the requirements for driving a “high side” N-channel MOS-gated power device are extremely similar to those in a high side switch, as are the issues concerning P-channel devices. In addition, a P-channel device with characteristics that match those of the low-side N-channel device may not be available.

Background: “MOS-Gated” Power Devices

A wide variety of power semiconductor devices have been proposed. Many make use of voltage-controlled conduction in a channel region in combination with other mechanisms, and such devices, as well as MOS transistors, are collectively referred to herein as “MOS-gated” devices. One very simple example is the “IGBT”, in which minority carriers are injected at the drain end of the channel to increase total current for a given flow of electrons. However, there are many other MOS-gated power devices (including MCTs, IGTHs, and many others), as described e.g. in B. J. Baliga, POWER SEMICONDUCTOR DEVICES (1995), which is hereby incorporated by reference.

Background: High-Side Switching

A high-side switch, where the switch is between the more positive power supply terminal and the load, is required in many applications. One such application is motor switching. From a circuit design perspective, a P-channel device would be easiest to drive and a logical choice for high-side switches. However, N-channel devices are generally used because they have better transconductance and therefore are smaller in area than P-channel devices. Unfortunately, N-channel devices, when operated in the common-drain configuration, may require a gate voltage that is greater than the supply voltage. If the voltage on the drain (or equivalent) terminal is the highest voltage in the circuit, a drive voltage that is greater than the voltage on the drain (or equivalent) terminal is usually needed to turn the switch “on.” This requires use of a charge pump, additional voltage supply, bootstrap techniques, or other scheme to create the necessary voltage. FIG. 2 shows an example of prior art high-side switching with an N-channel enhancement device Q_(NE) and a charge pump CP. The charge pump CP is required to create a gate drive voltage for Q_(NE) that is higher than the +VCC supplied to the drain of Q_(NE).

Various drive techniques for high side n-channel MOS-gated power devices have been developed. These techniques include charge pumps, independent high-side power supplies, and bootstrap techniques (see “Drive Techniques for High Side N-channel MOSFETs” by Warren Schultz, Power Conversion & Intelligent Motion, June, 1987, which is hereby incorporated by reference.)

The document “INT 978,” hereby incorporated by reference, available from the International Rectifier web site discusses the gate drive requirements of high-sided MOS-gated devices. Table 1 in INT 978 discusses the following five gate drive methods for high-side devices: Floating Gate Drive Supply, Pulse Transformer, Charge Pump, Bootstrap, and Carrier Drive.

High-Side Switch with Depletion-Mode Device

The presently preferred embodiment incorporates a depletion-mode n-channel MOS-gated device as a high-side switch. The depletion-mode N-channel MOS-gated switch has its threshold voltage shifted so it is fully “on” with a gate-to-source voltage of zero volts. The use of normally “on” MOS-gated devices allows the use of a negative switching voltage for turning the device “off” rather than the prior art method which requires a switching voltage that is higher than the source voltage.

The MOS-gated high-side switch described herein can be used in any high-side switch application. Typical applications: include high-side switches in automotive and similar applications.

The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:

high-side semiconductor switching without driver charge pumps, independent high-side power supplies, or bootstrap techniques

all switches can be integrated onto an IC if the voltage capability of the IC technology exceeds the peak voltage

negative voltage control of the high-side switch

may be switched directly, allowing controllers or microprocessors to determine the state of the switch

BRIEF DESCRIPTION OF THE DRAWING

The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:

FIG. 1 shows a cross section of a typical N-channel vertical DMOS transistor.

FIG. 2 depicts a circuit with an N-channel MOS transistor used as a high-side switch and a charge pump to drive its gate.

FIG. 3 shows a high-side switch incorporating an N-channel depletion-mode MOS-gated device.

FIG. 4 shows the operation of the circuit of FIG. 3.

FIG. 5 depicts a high-side switch incorporating an N-channel depletion-mode MOS-gated device with a circuit for more rapid gate turn-off.

FIG. 6 shows the operation of the circuit of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment (by way of example, and not of limitation). Other circuits and devices may enjoy the benefits of this invention without the use or application of the details of the preferred embodiment.

A High-side Switch Incorporating an N-channel Depletion-mode Mos-gated Switch

A presently preferred embodiment of a high-side N-channel depletion-mode MOS device is shown in FIG. 3. Q_(1′)is the normally “on” N-channel MOS-gated device with a source voltage above ground. A negative voltage is used to turn off Q_(1′). Q₄ is the transistor used to supply the drive voltage to the gate of Q_(1′). D1 is a Zener diode with a breakdown voltage larger than the voltage required to bias Q_(1′) “off,” but with a breakdown voltage well below the gate-to-source breakdown voltage of Q_(1′). R is a resistor that provides a current flow path between the source and gate of Q_(1′). The voltage “−V” is a negative voltage sufficiently large to turn Q_(1′) “off” when Q₄ is turned “on.” R_(L) is a load resistance. In this embodiment, resistor R and the gate capacitance of Q_(1′) determine the turn-on speed of Q_(1′) when V_(in) switches from −V to 0V. This circuit turns “off” rapidly but turn-on speed can be hindered by the value of resistor R and the gate capacitance of Q_(1′). The circuit operates as shown in the table of FIG. 4.

In an alternate, and less preferred embodiment, Q₄ can be replaced by a mechanical switch. It should be obvious to one of ordinary skill in the art that many types of switches, semiconductor or mechanical, could be substituted for the semiconductor switch Q₄ shown in FIG. 3.

Generally, the included tables give specific values for some of the voltage and timing parameters just referred to, as used in the specific context of the presently preferred embodiment. It must be understood that these specific values are given merely to provide a wealth of detail regarding the preferred embodiment, and for better understanding of FIGS. 3, and 5, and do not by any means delimit necessary features of the invention.

High-side Switch Incorporating N-channel Depletion-mode Mos-gated Switch and Current Source for Fast Switching

The turn-on speed of the embodiment disclosed in FIG. 3 can be increased by using a current source, as shown in FIG. 5. In the embodiment shown in FIG. 5, resistor R between the gate and source of Q_(1′″) is replaced by a normally “on” MOS-gated device with a threshold voltage typically between 0V and −10V. R_(GB) Provides the gate-to-source voltage to bias Q₃ “on” or “off.” As Q₁₅ makes the transition from “off” to “on,” Q₃ provides the current for a rapid transition. The circuit operates as shown in the table of FIG. 6.

Integration of the High-Side Switch in a Single I.C.

The components in FIGS. 3 and 5 may be integrated onto a single piece of silicon as long as the peak voltage that will be present does not exceed the voltage that the silicon can withstand. This feature means that both the power switches and the sense and control circuits may be fabricated simultaneously, taking advantage of the, economic and other aspects of IC technology.

According to a disclosed class of innovative embodiments, there is provided: A circuit comprising: an N-channel drive element, connected to selectably source current from a positive reference voltage to a load node, to thereby power local ground-connected devices which may be connected to said load node; said drive element comprising a depletion-mode power device, which remains “on” whenever any voltage above local ground is applied to a gate terminal thereof; and circuitry configured to drive said gate terminal of said drive element below local ground to controllably turn “off” said drive element, whereby current is turned off to said load node.

According to another disclosed class of innovative embodiments, there is provided: A circuit comprising: a drive element, connected to selectably source current from a positive reference voltage to a load device which is also connected to local ground; said drive element comprising an N-channel depletion-mode power device, which remains at least partially on whenever any voltage above local ground is applied to a gate terminal thereof; and circuitry configured to drive said gate terminal of said drive element below local ground to controllably turn off said drive element, whereby current is turned off to said load element.

According to another disclosed class of innovative embodiments, there is provided: A circuit comprising: a first MOS-drive element, connected to selectably source current from a positive first reference voltage to a load device; and a second drive element, connected to selectably source current from a second reference voltage lower than said positive reference voltage to a gate of said MOS-drive element; said first MOS-drive element comprising an N-channel depletion-mode power device.

Definitions

Following are short definitions of the usual meanings of some of the technical terms which are used in the present application. (However, those of ordinary skill will recognize whether the context requires a different meaning.) Additional definitions can be found in the standard technical dictionaries and journals.

Body

The region whose surface is inverted to form a “channel” when carriers flow in the transistor in normal operating conditions. The material within which a transistor channel is formed.

Carrier

In a semiconductor, an electron or hole, which can move around in the semiconductor material to transport charge. The movement of carriers is how electrical current flows.

Depletion

Reduction of carrier density, in a volume of semiconductor material, due to applied voltage.

Depletion Mode

An active semiconductor device that is normally “on” with no control signal present.

Diffusion

The process of diffusion is the spontaneous movement of dopant or impurity atoms through a semiconductor, at a rate which depends on temperature and on the particular elements involved. The noun “diffusion” usually refers to a doped portion of a semiconductor material.

Diode

A two-terminal device. Typically a diode is rectifying, i.e. passes current in only one direction. A rectifying diode may be implemented, for example, by a P-N junction in a semiconductor material.

Dopant

An atom added to a semiconductor, which, when activated, provides a “carrier” (i.e. an electron or hole) which can move around in the semiconductor to enable the flow of current. For example, in silicon technology, boron or gallium can act as P-type dopants (or “acceptors”), and phosphorus, arsenic, or antimony can act as N-type dopants (or “donors”).

Drain

The region where the carriers terminate when current is flowing in the transistor in normal operating conditions.

Drift Region

In a DMOS transistor, the semiconductor region between the channel and drain.

Drive Element

Any active semiconductor device.

Enhancement Mode

An N-channel MOS transistor begins to conduct when its gate voltage rises above a certain “threshold” voltage (which is determined by the compositions and dimensions of any particular device). If the threshold voltage is positive, the transistor is normally “off”, and is referred to as an “enhancement-mode” device.

Field-Effect Transistor (FET)

A transistor in which the voltage on a gate terminal controls the density of majority carriers in a semiconducting channel region which is part of the conduction path.

Gate

An electrode that covers the surface of the body region between the source and the drain regions, and controls the carrier flow between the source and the drain of the transistor in normal operating conditions. In a field-effect transistor, the electrode to which a control voltage is applied to modulate the conduction of the transistor.

Gate Terminal

A control lead to a semiconductor device.

High-side Switch

A switch that is between the more positive power supply terminal and the load.

Junction

An interface between p-type and n-type semiconductor material.

Load Node

A point in a circuit where an electronic load may be connected.

MOSFET

An insulated-gate field-effect transistor. A MOSFET may be configured to be a lateral, vertical, or grooved vertical structure. A MOSFET generally has four regions: source, drain, gate, and body. A MOSFET operates as a switch which is controlled (at least partially) by the voltage on its gate.

N-channel

A channel of n-type semiconductor material induced in a FET as a result of a bias applied to the gate. This channel allows current to flow from the drain to the source of an NMOS transistor. Typically an N-type channel is formed by surface inversion of p-type material, but it may also be formed by surface enhancement of n-type material.

N-type

A volume of semiconductor which normally includes an excess of electrons. This can be achieved by introduction of “donor” dopants (such as phosphorus, arsenic, or antimony in silicon).

“On” State

a state in a semiconductor device wherein there is conduction between an input terminal such as a drain, and an output terminal such as a source.

“Off” State

a state in a semiconductor device wherein there is little conduction between an input terminal such as a drain, and an output terminal such as a source.

P-channel

A channel of p-type semiconductor material induced in a FET as a result of a bias applied to the gate. This channel allows holes to flow from the source to the drain of a PMOS transistor.

P-type

A volume of semiconductor which normally includes an excess of holes. This can be achieved by introduction of “acceptor” dopants (such as boron or gallium in silicon).

PMOS

A p-channel field effect transistor, or a circuit or chip containing this type of transistor.

Reference Voltage

A substantially constant voltage such as a power supply voltage, any power rail, a constant voltage from a diode or a band gap regulator, or ground.

Resistivity

A measure of the resistance of a given material. Units are ohms times length.

Semiconductor

A material which is less conductive than a metallic material, but more conductive than an insulator. (More precisely, a semiconductor will have a nonzero “bandgap” between its valence and conduction bands, which is no more than a few electron volts at the very most.) The most frequently used semiconductor material is silicon, but there are many others, including gallium arsenide (or “GaAs”), silicon-germanium, mercury cadmium telluride, indium phosphide, gallium-indium arsenide-phosphide, and silicon carbide.

Sheet Resistance

The resistance of a square resistor, made from a given thin-film material, which has contacts on two opposite sides.

Source

The region where carriers originate when current is flowing in the transistor in normal operating conditions.

Threshold Voltage

The voltage at which a transistor starts to turn on.

Modifications and Variations

As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given. For example, hybrid switches that are not constructed entirely of MOS (such as an IGBT) might be substituted for MOS devices where appropriate.

As another example, to reduce the number of external components, the depletion mode device Q3 (or its equivalent) in FIG. 5 may be integrated into the structure of the normally “on” DMOS high-side switch Q1.

While the majority of the circuits discussed in this disclosure describe normally “on” N-channel DMOS FETs, other MOS-gated devices such as IGBTs, MCTs, and other devices can be manufactured with normally “on” characteristics and can be used in these circuits.

Though N-channel normally “on” MOS-gated devices are described in this disclosure, it is possible to manufacture normally “on” P-channel MOS-gated devices.

Although the present invention may be implemented in a DMOS process, important aspects of this invention may also be obtained without DMOS process techniques, as long as the process uses a MOS gate.

Additionally, even though the “M” in MOS originally referred to “Metal,” those of ordinary skill in the art understand that the gate may be constructed of any conducting material. In a similar fashion, although the “O” in MOS originally stood for oxide, this “O” layer may be any insulator above the channel region.

The following background publications provide additional detail regarding possible implementations of the disclosed embodiments, and of modifications and variations thereof. All of these publications are hereby incorporated by reference: B. Jayant Baliga, “Trends in Power Semiconductor Devices”, IEEE Transactions on Electron Devices, Vol. 43, No. 10, October, 1996; Bismal K. Bose, Power Electronics and Variable Frequency Drives, IEEE Press, 1996; David Bruce, Modern Electronics, Reston Publishing, 1984; Thomas M Frederiksen, Intuitive IC Electronics, McGraw-Hill, 1989; Mohan et al., Power Electronics, John-Wiley and Sons, 1995.

None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: THE SCOPE OF PATENTED SUBJECT MATTER IS DEFINED ONLY BY THE ALLOWED CLAIMS. Moreover, none of these claims are intended to invoke paragraph six of 35 USC section 112 unless the exact words “means for” are followed by a participle. 

What is claimed is:
 1. A circuit comprising: a positive reference voltage; an N-channel drive element connected to said positive reference voltage and to a load node, to selectably source current from said positive reference voltage to said load node thereby powering local ground-connected devices which may be connected to said load node; said drive element comprising a depletion-mode power device having an established threshold voltage that is less than the positive reference voltage and a gate, the drive element being turned “on” whenever any voltage more positive than the established threshold voltage is applied to the gate; and circuitry configured to drive said gate of said drive element more negative than the established threshold voltage to controllably turn “off” said drive element, whereby current is turned off to said load node.
 2. The circuit of claim 1, wherein said drive element is DMOS.
 3. The circuit of claim 1, wherein said drive element is connected between, and in series with, said reference voltage and said load node.
 4. A circuit comprising: a positive reference voltage; a drive element connected to said positive reference voltage and to a load element through a load node to selectably source current from said positive reference voltage to said load element, said load node having a second voltage. a local ground connected to said load element; said drive element comprising an N-channel depletion-mode power device, having a gate and an established threshold voltage that is less than the positive reference voltage, the drive element remaining at least partially “on” whenever any voltage more positive than second voltage is applied to the gate and the second voltage is at least equal to or greater than the threshold voltage; and circuitry means for driving said gate of said drive element more negative than the established threshold voltage and the second voltage to controllably turn “off” said drive element, whereby current is turned off to said load element.
 5. The circuit of claim 4, wherein said drive element is DMOS.
 6. The circuit of claim 4, wherein said drive element and said circuitry are incorporated into a single integrated circuit.
 7. The circuit of claim 4, wherein said circuitry incorporates a microcontroller.
 8. A circuit comprising: a reference potential; a first reference voltage than is more positive than the reference potential; a second reference voltage that is more negative than the reference potential; a first MOS-drive element, connected to selectably source current from the first reference voltage to a load device, the load device having a negative terminal connected to the reference potential; and a second drive element, connected to selectably source current from the second reference voltage to the gate of said first MOS-drive element; said first MOS-drive element comprising an N-channel depletion-mode power device.
 9. The circuit of claim 8, wherein said first drive element is DMOS.
 10. The circuit of claim 8, wherein said second drive element is DMOS.
 11. The circuit of claim 8, wherein said second drive element comprises an N-channel device.
 12. The circuit of claim 8, wherein said first drive element is a depletion-mode device.
 13. The circuit of claim 8, wherein said second drive element is a depletion-mode device.
 14. The circuit of claim 8, wherein said first drive element is an insulated-gate device with a doped gate insulator.
 15. The circuit of claim 8, wherein said drive elements have a space charge layer with electron mobility more than twice hole mobility.
 16. The circuit of claim 8, wherein said first drive element and said second drive element are incorporated in a single integrated circuit.
 17. The circuit of claim 1, wherein said drive element and said circuitry are incorporated into a single integrated circuit.
 18. The circuit of claim 1, wherein said circuitry incorporates a microcontroller.
 19. A circuit comprising: a first reference potential; a first reference voltage that is more positive than the reference potential; a load having a first terminal connected to the reference potential; an N-channel drive element having an established threshold voltage that is less than the first reference voltage and a gate, the drive element being connected to said positive reference voltage and to the load at a second terminal to source current from said positive reference voltage to said load whenever a first voltage is applied to the gate and to cease sourcing current from the positive reference voltage to the load whenever a second voltage is applied to the gate, the first voltage being more positive than the threshold voltage and the second voltage being more negative than the threshold voltage; and circuitry configured to selectably provide said gate of said N-channel drive element the first and second voltages.
 20. The circuit of claim 19, wherein said threshold voltage is more positive than the reference potential.
 21. The circuit of claim 19, wherein said threshold voltage is more negative than the reference potential.
 22. The circuit of claim 19, wherein said threshold voltage is equal to the reference potential. 